The present invention relates to nuclear fuel bundle assemblies and, particularly, to dry-storage of fuel bundle assemblies.
The core of a boiling water nuclear reactor (BWR) includes a plurality of fuel bundle assemblies arranged side-by-side. When removed from the core of a BWR, the fuel bundle assemblies are temporarily stored in a water-filled spent fuel pool. After years of storage in the spent fuel pool, the fuel bundle assemblies are transferred to a dry storage containment vessel, e.g., a canister. Typical dry storage canisters hold approximately sixty (60) BWR fuel bundle assemblies and are backfilled with helium to provide a convective heat transfer medium to remove heat from the assemblies to the canister.
The loading configurations in dry storage canisters are controlled to ensure decay-heat-load and criticality limits are met. Decay heat loads are controlled to limit internal temperatures that could damage sound, i.e., undamaged, fuel bundles or degrade damaged fuel bundles. Decay heat can be transferred from the fuel bundle assemblies to the dry storage canister via conduction, convection and radiation. Criticality calculations are performed to ensure a criticality cannot occur in a hypothetical reloading accident. In these criticality calculations sound fuel bundles are assumed to remain sound while damaged fuel bundles are assumed to fail completely.
The determination of whether a fuel bundle is sound or damaged is made prior to loading the dry storage containment vessel or canister. A fuel bundle is considered damaged if the cladding on the fuel rods has been breached such that fuel particles could be released. Fuel particles are highly radioactive and if released in large scale, the criticality assumptions would be modified.
Therefore, damaged fuel bundles are not placed directly in dry storage like sound fuel bundles. The damage to cladding on fuel rods may allow fuel particles to fall into the fuel bundle assembly and into the dry storage containment vessel, unless the released fuel particles are confined to the damaged fuel bundle assembly. The minimum particle size required to be confined is approximately 1 mm (millimeter). Because of the potential for fuel particle release, damaged fuel bundles are conventionally placed within a damaged fuel storage canister prior to being placed within the dry storage containment vessel.
A conventional storage approach is to remove the channel from each damaged fuel bundle assembly and place the remaining fuel bundle within a damaged fuel storage canister. The canister slides over the length of the fuel bundle and is capped at each end. The caps on the canister prevent fuel particles from flowing out of the ends of the canister and into the dry storage containment vessel. The caps are vented to allow the fluid pressures within the canister and around the fuel bundle to equalize with the pressure in the containment vessel but are not designed to promote gas flow that enhances convective heat transfer.
The damaged fuel storage canister is placed in the dry storage containment vessel with sound fuel bundle assemblies and other canisters having damaged fuel bundles. The number of damaged fuel storage canisters in a dry storage containment vessel is limited based on either the heat load or criticality requirements. After being removed from the damaged fuel bundle assembly, the channel is discarded in an alternate waste stream.
This typical approach to storing damaged fuel bundle assemblies has at least three disadvantages: (1) the damage fuel bundle assembly must be partially disassembled by removing the channel; this results in fuel moves without the protection of the channel and considerations of hypothetical bundle drop accidents in the fuel pool, (2) The added costs for a damaged fuel storage canister and to discard the channel in an alternate waste stream, and (3) convection in the damage fuel canister is not considered in the heat load determinations.